Basement excavation: How to build down

Digging out a basement can add quality living space to your home

By Ian Harvey, Toronto Star

Basement excavation and underpinnings aren’t for the feint of heart. As renovations go, it’s one of the most technically complex construction techniques to execute because, done wrong, it can literally bring the house down. Also, the homeowner is liable for damage caused to neighbour’s homes as well, so it’s wise to do it right and ensure insurance coverage and hire a qualified, experienced contractor.

Turning that dingy, damp stone-age space into a quality living or activity area is fairly easy if there’s full height between the floor and ceiling, of course. The trick is taking homes built before 1950 or so and adding headroom.

Typically, Michael Upshall of Probuilt, a Toronto-based contractor specializing in basement underpinning, sees homes with basements with around six feet of head room with encroachments by ductwork and other mechanicals (plumbing, wiring). Others have even less headroom and are mere crawlspaces.

“The key to successful underpinning of any basement project is planning,” says Upshall, noting there are a few key technical things to keep in mind for those considering an excavation, including making sure the original foundations are good. The important thing is not to stress the existing foundations all at once, says Upshall, and that involves having an engineer determine what condition the footings are in (the base on which the home’s walls are set below ground) and how much weight they are bearing.

Next is to look at the heating and cooling system to see if it should be moved out during the work or whether the contractor should dig around it. In some cases, the furnace or boiler may be due for replacement so everything gets ripped out. The plumbing should also be checked for the same reasons and then the digging can start.

“We dig out the main part of the basement leaving the perimeter,” says Upshall, noting this is where much of the costs start to add up. Digging by hand is expensive because it’s labour intensive.

The best plan is to get a machine down there.

“We can set up a conveyor belt system but sometimes we just dig down outside and cut a hole in the basement wall and come in that way with a Bobcat (small front end loader), ” says Upshall, who says budgeting $30 to $40 a square foot (for just the excavation) is a good starting point.

Next, the perimeter floor is marked out in two- or three-foot sections.

“Each section is numbered – one, two, three and sometimes four, whatever the engineer directs,” he says. “Then we cut out each section numbered “one” all the way around leaving the spaces between untouched.”

This way, there are still two or three other sections holding up the walls and the crews can dig down to the required level, set in a concrete form and pour new footings.

“We don’t go all the way up to the existing footings, we leave a gap of a few inches,” says Upshall. “Then we come back with a special epoxy grout – it’s $39 a bag – and we hand pack it up to the footing. It also has to be inspected (by the city) at each stage.”

It’s done this way, he explains, because with the amount of dirt in a basement, there’s a risk of contaminating the concrete.

Then, the next numbered section is excavated and the process repeats itself until the entire perimeter is dug out and the footings underpinned.

There are a couple of alternatives to underpinning: one is benching, which involves pouring a mass of concrete away from the wall at the original floor point, about one unit out for each unit down. Therefore, if the basement was to be excavated two feet, there would be a corresponding two-foot by two-foot “bench” intruding into the basement. It’s cheaper and faster and the benches can be hidden by closets or even kitchen-style cabinets.

For new homeowners, the best time to renovate is early on, so you can enjoy your home improvements and experience the least disruption to your routine. But can you afford to renovate right after paying your down payment, closing costs and moving expenses?